Method of operating a heat exchanger

ABSTRACT

A method of operating a heat exchanger in which a liquid medium is passed into heat-exchanging relationship with a heat-exchanger wall surface, wherein inert solid particles in a particle size range of 10 to 150 microns are added to the liquid medium in an amount of 0.5 to 20% by volume to form a suspension in which the Reynolds number Re uD/v is 10,000 to 70,000 for a diameter D of 1 cm.

United States Patent 119- Plass 1111 3,886,997 1 June 3, 1975 METHOD OF OPERATING A HEAT EXCHANGER [75] Inventor:

[73] Assignee:

Ludolf Plass, Kronberg, Germany Metallgesellschaft Aktiengesellschaft, Frankfurt am Main, Germany 22 Filed: 01:1. 9, 1973 211 App]. N0..' 404,680

[30] Foreign Application Priority Data Oct. 6, I972 Germany 224899l4 521 13.8.0 165/1; 126/4 w; 165/104 151 1111.0. F28d 13/00 [58] Field 6: Search 165/1, 104, 101, 5, 95;

[56] References Cited UNITED STATES PATENTS 3,127,936 4/1964 Eurenius l26/4 W RESERVOIR 3,596,7l3 8/l97l Kaitz 126/4 W Primary ExaminerCharles Sukalo Attorney, Agent, or Firm-Karl F. Ross; Herbert Duhno [57] ABSTRACT A method of operating a heat exchanger in which a liquid medium is passed into heat-exchanging relationship with a heat-exchanger wall surface, wherein inert solid particles in a particle size range of l0 to 150 microns are added to the liquid medium in an amount of 0.5 to 20% by volume to form a suspension in which the Reynolds number Re LTD/v is 10,000 to 70,000 for a diameter D of l cm.

6 Claims, 6 Drawing Figures PART! C LE N0 PPS? FEEDEE PRIICLE HOPPL'R PATEHTEU M522 3 \975 SHEET 1 neg-um VOL% SOLIDS HEAT- 0 TRANS FEE W-TE -UFFIJUH I975 SHEET 3 VOL souos PATENTEDJUH 31975 $886,997

SHEET 6 PAPT l C LE HOPPEe F'E'E DEE PAPf/CLE HOPPEI? FEED El? FIG. 6

RESERVOIR METHOD OF OPERATING A HEAT EXCHANGER FIELD OF THE INVENTION This invention relates to a method of operating closed heat exchangers which are supplied with liquid heat-exchange fluids.

BACKGROUND OF THE INVENTION The rate at which heat is transferred from a given liquid heat-exchange fluid to a wall and vice versa depends substantially on the velocity of the fluid and on the area and nature of the wall surface in contact with the heat-exchange fluid. The heat-transfer rate can be increased by an increase of the velocity of the heat exchange fluid, an increase of the area of the wall (eg by the incorporation of baffles) and by a toughening of the wall surface. Each of these measures gives rise to a rapidly increasing pressure loss. For instance, an increase of the velocity of the fluid will result in an increase of the pressure loss which is almost twice as large as the increase of the heat-transfer rate so that the increase of the velocity of the fluid is limited by economic considerations. The incorporation of baffles and the roughening of the heat-exchange wall surface also result in high pressure losses.

OBJECT OF THE INVENTION It is an object of the invention to improve the heat transfer rate in closed heat exchangers without signifi' cant structural expenditure and without a need for an expensive mode of operation, particularly without a high pressure loss.

SUMMARY OF THE INVENTION To accomplish this object, the method of operating closed heat exchangers supplied with liquid heat exchange fluids according to the invention provides that inert solids having an average particle diameter in the range of l-l50 microns are incorporated in the heat exchange fluid in an amount of 0.5% by volume, and the average velocity of the suspension is adjusted to correspond to a Reynolds number in the range of l0,000-70,000 based on a pipe diameter of l centimeter.

It is known that the rate at which heat is transferred from a gas to the wall and vice versa can be increased by an addition of particulate solids to the gas (Chem. Ing. Techn. 39l967, page 282) because the addition of solids having a high specific heat to gas having a low specific 'heat results in a gas-solid mixture having a higher specific heat, which is believed to be responsible for the increase of the heat-transfer rate. Where liquid heat-transfer fluids are used which always have a higher specific heat than the added solids, a liquid-solid mixture will result which has a lower specific heat than that of the original liquid so that a decrease of the heattransfer rate would be expected. My experiments have surprisingly shown, however, that considerable improvement can be achieved, contrary to the expectations, if the requirements of the invention are fulfilled.

Reynolds number is defined as Re H D/v where E is the average velocity in the main direction of flow in meters per second, D is a length, in meters. which de fines the cross-sections of the passages of the heat exchanger which are traversed, e.g., the pipe diameter, and v is the kinematic viscosity in m sec". For this reason the dimensions of the passages of the heat exchanger must be taken into account in selecting the Reynolds number range which meets the requirements of the invention. As applied to tubular passages having a diameter which differs from the unit value of l centimeter. the Reynolds numbers must be linearly changed accordingly. Where tubes are used which are for example 2 or 3 centimeters in diameter, the Reynolds number must lie between 20,000 and l40.000 or between 30,000 and 210,000, respectively.

The increase of the heat-transfer rate will be particularly large if, according to a preferred feature of the invention, the inert solids incorporated in the heatexchange fluid have an average particle diameter of 10-50 microns and are used in an amount of l6% by volume, particularly 2.56% by volume.

The solids to be added to the liquid heat-exchange fluid should suitably be of medium hardness. Excessively high hardness could result in damage to the wall surface of the heat exchanger. An insufficiently high hardness could result in a progressively increasing reduction of the particle size of the added solids. Particularly suitable solids are those having a density of more than 1 gram per cubic centimeter, such as sand, ore dust, and ground slag.

Suitable heat-exchange fluids for the purposes of the invention are water, aqueous solutions, e.g. of acids, bases, and salts, or organic solvents, such as diphenyl and mineral oil, provided that the viscosity does not exceed ten times that of water at the temperatures at which the fluid is used The method according to the invention can be used in virtually all closed heat exchangers. These include, double-tube heat exchangers, annular-gap heat exchangers, plate heat exchangers and spiral heat exchangers. Particularly suitable are the heat exchangers which have heating or cooling passages that are free from baffles. The passages may have any desired crosssection.

The heat-exchange fluid flowing through the heat exchangers may be conducted in a closed or open cycle. An open cycle will be obtained if the heat exchange fluid is partly evaporated by being subjected to a pressure relief when it has flown through the heat exchanger.

DESCRIPTION OF THE DRAWING The above and other objects, features and advantages of the present invention will become more readily apparent from the following description, reference being made to the accompanying drawing in which:

FIGS. 1 and 2 are graphs illustrating the dependence of the heat-transfer rate upon the proportion of particles in the liquid medium;

FIGS. 3-5 are graphs illustrating the dependence of the heat-transfer rate upon the Reynolds number of the suspension; and

FIG. 6 is a diagram of a heat-exchanger system embodying the invention.

SPECIFIC DESCRIPTION An example of a heat-exchange system according to the present invention has been illustrated in FIG. 6. The tube-bundle heat exchanger 10 has a pair of tube sheets 11, 12 bridged by the tubes 13 from which the first heat-exchange liquid is drawn at chamber 14 and fed by pipe 16 to a pump 17. Particles may be added to the liquid from a hopper 18 by a metering device 19.

The suspension. homogenized by the pump 17, is passed through a heat source 20 and is supplied by pipe 21 to the chamber whence the liquid traverses the tubes 13 in a closed system.

The other heatexchange liquid is drawn from a reservoir 27 by a pump 26 and receives particles from a hopper 28 by a metering device 29. The suspension is delivered by a pipe 25 to the inlet 22 of the heatexchange space around the tubes 13 and, having been heated, passes through outlet 23 into a heat consumer 24. If the latter evaporates the second liquid, the particles are returned to the hopper 28 as represented at 30.

SPECIFIC EXAMPLES The experiments were conducted with a heat exchanger having tubular cooling passages 2.7 centimeters in diameter, and with water at 50C. as a heat exchange fluid. The inert solids consisted of sand in various particle sizes.

FIGS. 1 and 2 show sets of curves which indicate the change of the heat transfer rate in percent in dependence on the amount of added solids in per cent by volume for given average particle diameters and Reynolds numbers (ReW). 1n the experiments represented in FIGS. 1 and 2, the tested systems had Reynolds numbers of 88,000 and 164,000, respectively. The inert solids consisted of quartz sand which had average particle diameters of 12, 25, 40, 70 and l microns, respectively.

FIGS. 3, 4, and 5 indicate the change of the heat transfer rate in dependence on the solids content for Reynolds numbers of 88,000, ll3,000, l40,000. and 164,000 for solids having average particle diameters dp of 12, 40, and 70 microns, respectively. The average velocities of the heat exchange fluid (1.5, 2.0, 2.5, 3.0 meters per second) are stated beside the Reynolds numbers.

It is apparent from the diagram that the solids content, the mean particle diameter, and the Reynolds number are parameters which by complicated relations are responsible for the change of the heat-transfer rate. It is remarkable that the optimum increase of the heattransfer rate depends to a comparatively small degree on the concentration of the solid and in the cases which were investigated was achieved with concentrations between about 3.5 and 58% by volume. On the other hand. the average particle, diameter, particularly ifit is small. and the Reynolds number have a strong influence. The Reynolds number must be increased as the average particle diameter decreases. and decreased as the average particle diameter increases. The highest increase measured during the experiments was almost 34% and was obtained with an average particle diameter of 25 microns, a solids concentration of about 3.5% by volume, and a Reynolds number of 164.000.

The maximum increase of the pressure loss resulting in the experiments when inert solids were added in the preferred concentration range of l67? by volume was 8%.

With reference to the informative data represented in FIGS. l to 5, the optimum conditions for specific heat exchange operations can be determined in a simple manner.

I claim:

1. In a method of operating a heat exchanger wherein a liquid heat-exchange fluid is passed in contact with a heat exchanger surface, the improvement which com prises suspending in said fluid particulate solids physically and chemically inert to said liquid heat'exchange fluid with an average particle size of 10 to microns in an amount of 0.5 to 20% by volume and such that the suspension has a Reynolds number of 10,000 to 70,000 at the average velocity of the liquid heat-exchange fluid in contact with said surface based on a flow channel of a diameter of l centimeter.

2. The improvement defined in claim 1 wherein said solids have an average particle diameter of 10 to 50 microns.

3. The improvement defined in claim 2 wherein said solids are incorporated in the liquid heat-exchange fluid in an amount of l to 6% by volume.

4. The improvement defined in claim 3 wherein said solids are incorporated in the liquid heat-exchange fluid in an amount of 2.5 to 6% by volume.

5. The improvement defined in claim 4 wherein said liquid heat-exchange fluid has a viscosity of at most 10 times that of water at the operating temperature.

6. The improvement defined in claim 5 wherein said solids are sand, ore dust or ground slag. 

1. IN A METHOD OF OPERATING A HEAT EXCHANGER WHEREIN A LIQUID HEAT-EXCHANGE FLUID IS PASSED IN CONTACT WITH A HEAT EXCHANGER SURFACE, THE IMPROVEMENT WHICH COMPRISES SUSPENDING IN SAID FLUID PARTICULATE SOLIDS PHYSICALLY AND CHEMICALLY INERT TO SAID LIQUID HEAT-EXCHANGE FLUID WITH AN AVERAGE PARTICLE SIZE OF 10 TO 150 MICRONS IN AN AMOUNT OF 0.5 TO 20% BY VOLUME AND SUCH THAT THE SUSPENSION HAS A REYNOLDS NUMBER OF 10,000 TO 70,000 AT THE AVERAGE VELOCITY OF THE LIQUID HEAT-EXCHANGE FLUID IN CONTACT WITH SAID SURFACE BASED ON A FLOW CHANNEL OF A DIAMETER OF 1 CENTIMETER.
 1. In a method of operating a heat exchanger wherein a liquid heat-exchange fluid is passed in contact with a heat exchanger surface, the improvement which comprises suspending in said fluid particulate solids physically and chemically inert to said liquid heat-exchange fluid with an average particle size of 10 to 150 microns in an amount of 0.5 to 20% by volume and such that the suspension has a Reynolds number of 10,000 to 70,000 at the average velocity of the liquid heat-exchange fluid in contact with said surface based on a flow channel of a diameter of 1 centimeter.
 2. The improvement defined in claim 1 wherein said solids have an average particle diameter of 10 to 50 microns.
 3. The improvement defined in claim 2 wherein said solids are incorporated in the liquid heat-exchange fluid in an amount of 1 to 6% by volume.
 4. The improvement defined in claim 3 wherein said solids are incorporated in the liquid heat-exchange fluid in an amount of 2.5 to 6% by volume.
 5. The improvement defined in claim 4 wherein said liquid heat-exchange fluid has a viscosity of at most 10 times that of water at the operating temperature. 